The present invention relates to a method of producing an Rxe2x80x94Fexe2x80x94B type rare-earth magnet. More specifically, the present invention relates to a powder compacting apparatus that is particularly suitable for use with a rare-earth alloy powder having a reduced oxygen content, and a method of producing a rare-earth magnet using the same.
A rare-earth alloy sintered magnet is made by compacting a magnetic powder that has been obtained by pulverizing a rare-earth alloy, and then subjecting the product to a sintering step and an aging step. Currently, two types of rare-earth alloy sintered magnets are widely used in various fields: samarium-cobalt magnets and neodymium-iron-boron magnets. Particularly, neodymium-iron-boron magnets (hereinafter referred to as xe2x80x9cRxe2x80x94Fexe2x80x94B magnetsxe2x80x9d, wherein R denotes a rare-earth element and/or Yttrium, Fe denotes iron, and B denotes boron.) have been actively employed in various electronic devices because they exhibit the highest magnetic energy product among various magnets and are relatively inexpensive. An Rxe2x80x94Fexe2x80x94B magnet is primarily composed of a major phase of an R2Fe14B tetragonal compound, an R-rich phase of Nd, or the like, and a B-rich phase. Part of Fe may be substituted with a transitional metal such as Co or Ni, and part of B may be substituted with C.
In the prior art, such a rare-earth alloy has been made by an ingot casting method in which a material molten alloy is put in a mold and cooled at a relatively slow rate. An alloy made by the ingot casting method is crushed and pulverized through a known pulverization process. The obtained alloy powder is then compacted by any of various powder compacting apparatuses, and then transferred into a sintering chamber, where the compact (green compact) of the alloy powder undergoes a sintering step.
In recent years, rapid cooling methods such as a strip casting method and a centrifugal casting method have been attracting public attention, in which a molten alloy is contacted with a single roll, a pair of rolls, a rotating disc, a rotating cylindrical mold, or the like, so as to be cooled at a relatively high rate, thereby making a solidified alloy that is thinner than an alloy ingot. The rapidly cooled alloy thus obtained has a thickness of 0.03-10 mm. In an exemplary rapid cooling process, a chill roll in contact with a molten alloy is rotated so that the molten alloy is picked up by the roll in the form of a thin sheet on the roll surface. The solidification of the sheet of molten alloy on the chill roll starts from the plane along which the molten alloy contacts the chill roll (xe2x80x9croll contact planexe2x80x9d), wherein a columnar crystal starts growing from the roll contact plane in a direction perpendicular to the roll contact plane. As a result, a rapidly cooled alloy made by a strip casting method, or the like, has a composition containing an R2T14B crystal phase (wherein T denotes iron and/or a transition metal element substituting part of iron with Co, or the like) whose size in the short axis direction is between 0.1 xcexcm and 100 xcexcm and whose size in the long axis direction is between 5 xcexcm and 500 xcexcm, and an R-rich phase that exists dispersed along the grain boundaries of the R2T14B crystal phase. The R-rich phase is a non-magnetic phase having a relatively high concentration of rare-earth element R, and has a thickness (equivalent to the width of the grain boundary) less than or equal to 10 xcexcm.
A rapidly cooled alloy is made at a higher cooling rate (102-104xc2x0 C./sec) as compared with an alloy ingot made by a conventional ingot casting method (mold casting method), and therefore has advantageous characteristics such as a fine structure and a small crystal grain diameter. A rapidly cooled alloy is also advantageous in that it has a desirable R-rich phase dispersion as it has a large grain boundary area and the R-rich phase can exist thinly dispersed along the grain boundaries.
However, a magnetic powder of a rapidly-cooled alloy such as a strip-cast alloy is easily oxidized. It is believed that this is because the R-rich phase, which is easily oxidized, is likely to appear on the grain surface of a powder of a rapidly-cooled alloy. A powder of a rapidly-cooled alloy is very easily heated and ignited. Even if oxidization stops short of igniting the powder, the magnetic properties of the powder deteriorate significantly due to the oxidization.
While the heating and ignition of the rare-earth component due to oxidization occur also when compacting a rare-earth alloy powder that has been made by a conventional ingot casting method, the problem is more pronounced when compacting a powder of a rapidly-cooled alloy such as a strip-cast alloy.
In addition to the problem described above, the oxidization of a rare-earth alloy powder also causes a problem as follows.
It is known that the magnetic properties of an Rxe2x80x94Fexe2x80x94B magnet can be improved by increasing the content of the major phase, i.e., the R2Fe14B tetragonal compound. While a minimum amount of R-rich phase is required for a liquid phase sintering process, R also reacts with oxygen to produce an oxide, R2O3, whereby part of R is consumed for a purpose that has no contribution to sintering. Accordingly, an extra amount of R is required for the consumption by oxidization. The production of the oxide R2O3 increases as the amount of oxygen in the powder-making atmosphere increases. In view of this, attempts have been made in the prior art to reduce the amount of oxygen in the powder-making atmosphere and to reduce the relative amount of R in the final Rxe2x80x94Fexe2x80x94B magnet product, thereby improving the magnetic properties thereof.
Although it is preferred to reduce the amount of oxygen in a rare-earth alloy powder that is used to produce an Rxe2x80x94Fexe2x80x94B magnet, as described above, the method of reducing the amount of oxygen in a rare-earth alloy powder to improve the magnet properties has not been realized as a mass-producing technique for the following reason. When an Rxe2x80x94Fexe2x80x94B alloy powder is made under a controlled environment with a reduced oxygen concentration so that the amount of oxygen in the alloy powder is reduced to be less than or equal to 4000 mass parts per million (ppm), for example, the powder may violently react with the oxygen in the atmosphere and may ignite within a few minutes at room temperature. Thus, although it was understood that it would be preferred to reduce the amount of oxygen in the rare-earth alloy powder in order to improve the magnetic properties thereof, it was actually difficult to handle a rare-earth alloy powder with such a reduced oxygen concentration at a manufacturing site such as a plant.
Particularly, in a pressing step for compacting a powder, the temperature of the compact increases due to the frictional heat that is generated between powder particles being compacted and/or the frictional heat that is generated between the powder and the inner wall of the cavity when the compact is taken out of the cavity, thereby increasing the risk of ignition.
It has been proposed in the art to perform a compaction process in an inert gas atmosphere in order to suppress such an oxidization as disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-346102, which describes providing an airtight gas chamber which accommodates at least compacting apparatus including a pressing section and a powder supply section for supplying a powder to a powder feeding device.
However, the conventional compacting apparatus is uneconomical because the gas chamber has a relatively large volume, thereby requiring a large amount of inert gas to fill the gas chamber. In the conventional compacting apparatus, the inert gas is not supplied directly to the rare-earth alloy powder, and the space around the passageway via which the rare-earth alloy powder (or the compact) is transferred (e.g., the space around the powder feeding device) is also exposed to a high concentration of inert gas, thereby failing to effectively utilize the inert gas.
Moreover, in cases where the inside of the gas chamber is frequently exposed to the air atmosphere (e.g., where die replacement is frequently needed for making various types of compacts), the use of the conventional apparatus significantly reduces the productivity as it requires a long period of time for substituting the gas in the gas chamber with an inert gas each time a die is replaced by another.
Moreover, although the pressing step with a compacting apparatus is automated, the compacting apparatus requires frequent maintenance, and such maintenance often requires a human operator. If the compacting apparatus is placed in an inert atmosphere, an operator who comes close to the compacting apparatus for trouble shooting may suffer from atmospheric hypoxia. For these and other reasons, placing the entire compacting apparatus in an inert atmosphere is not a practical approach.
In the prior art, a liquid lubricant such as a fatty acid ester is added to a fine powder prior to the pressing step in order to improve the compressibility of the powder. Although such addition of a liquid lubricant forms a thin oily coating on the surface of the powder particles, it cannot sufficiently prevent the oxidization of the powder when a powder whose oxygen concentration is less than or equal to 4000 mass ppm is exposed to the atmospheric air.
In view of this, in the prior art, a slight amount of oxygen is intentionally introduced into the atmosphere during pulverization of a rare-earth alloy so as to slightly oxidize the surface of the finely pulverized powder, thereby reducing the reactivity thereof. For example, Japanese Patent Publication for Opposition No. 6-6728 discloses a technique of using a supersonic flow of an inert gas containing a predetermined amount of oxygen to finely pulverize a rare-earth alloy while forming a thin oxidized coating on the particle surface of the fine powder produced through the pulverization. With the technique, oxygen in the atmospheric air is blocked by the oxidized coating formed on the powder particle surface, thereby preventing the heating and ignition of the powder due to oxidization. However, the presence of the oxidized coating on the powder particle surface increases the total amount of oxygen contained in the powder.
Japanese Laid-Open Patent Publication No. 10-321451 discloses a technique of mixing a low-oxygen Rxe2x80x94Fexe2x80x94B alloy powder with a mineral oil, or the like, to obtain a slurry. Since the powder particles in the slurry are not exposed to the atmospheric air, it is possible to prevent the heating and ignition of the alloy powder while reducing the amount of oxygen contained therein.
However, this conventional technique leads to a poor productivity because, after filling the cavity of the compacting apparatus with an Rxe2x80x94Fexe2x80x94B alloy powder in the form of a slurry, it is necessary to perform the pressing step while squeezing the oil component out of the alloy powder.
It is therefore an object of this invention to provide a practical method of producing a rare-earth magnet that exhibits desirable magnetic properties without causing an accidental ignition even when using a rare-earth alloy powder that is easily oxidized.
Another object of the present invention is to provide a method of producing a rare-earth magnet in a safe and efficient manner while using a rare-earth alloy powder having a low oxygen concentration.
An inventive powder compacting apparatus includes: an airtight container capable of storing a rare-earth alloy powder therein; an airtight feeder box moved between a powder-filling position and a retracted position; and an airtight powder supply device capable of supplying the rare-earth alloy powder from the container into the feeder box without exposing the rare-earth alloy powder to an atmospheric air.
In a preferred embodiment, the powder compacting apparatus further includes means for supplying an inert gas into the powder supply device, whereby an oxygen concentration in an atmosphere in each of the powder supply device and the feeder box during a pressing operation is controlled to be 50000 volume ppm or less.
In a preferred embodiment, the powder compacting apparatus further includes at least one gas concentration sensor for sensing the oxygen concentration in the powder supply device.
In a preferred embodiment, the powder compacting apparatus further includes at least one temperature sensor for sensing a temperature of the rare-earth alloy powder in the powder supply device.
In a preferred embodiment, the powder compacting apparatus further includes at least one temperature sensor for sensing a temperature of the rare-earth alloy powder in the feeder box.
In a preferred embodiment, the powder supply device includes a non-flexible hollow portion and a flexible hollow portion; and an open/close means is provided between the non-flexible hollow portion and the flexible hollow portion, wherein the open/close means is closed in response to an increase in the temperature of the rare-earth alloy powder.
In a preferred embodiment, at least a portion of the powder supply device is made of a flexible hollow portion; and the flexible hollow portion can flexibly deform as the feeder box is moved.
In a preferred embodiment, a screw feeder for moving the rare-earth alloy powder toward the flexible hollow portion at a controlled rate is provided in the non-flexible hollow portion of the powder supply device.
In a preferred embodiment, the flexible hollow portion of the powder supply device is made of a two-layer hose.
In a preferred embodiment, a device for vibrating the flexible hollow portion of the powder supply device so as to facilitate falling of the rare-earth alloy powder through the flexible hollow portion is attached to the flexible hollow portion.
In a preferred embodiment, the powder supply device includes a material receptacle for receiving the rare-earth alloy powder from the container; and a connection section including a valve capable of closing the material receptacle is provided between the container and the material receptacle.
In a preferred embodiment, the container is detachably connected to the connection section.
In a preferred embodiment, the feeder box includes a level sensor for sensing an upper surface level of the rare-earth alloy powder in the feeder box; and the rare-earth alloy powder is supplied into the feeder box by the powder supply device when the upper surface level of the rare-earth alloy powder in the feeder box has decreased below a predetermined level.
In a preferred embodiment, an inside of a powder supply passageway of the powder supply device is an inert gas atmosphere; and an outside of the powder supply passageway is an air atmosphere.
An inventive method is a method of producing a rare-earth magnet by performing a compaction process using the powder compacting apparatus as described above, the method including the steps of: storing a rare-earth alloy powder in the container; operating the powder supply device to supply the rare-earth alloy powder from the container into the feeder box without exposing the rare-earth alloy powder to the atmospheric air; and producing a compact by pressurizing the rare-earth alloy powder supplied from the feeder box into a predetermined space.
In a preferred embodiment, a rare-earth alloy powder whose oxygen content is 4000 mass ppm or less is compacted.
In a preferred embodiment, the method further includes the steps of: taking a compact made by the compacting apparatus out of the compacting apparatus and then impregnating the compact with an oil agent; and sintering the compact.
In a preferred embodiment, the method further includes the step of mixing the rare-earth alloy powder with a lubricant.
In a preferred embodiment, the rare-earth alloy powder is a dry powder.
Another inventive method of producing a rare-earth magnet includes the steps of: supplying a rare-earth alloy powder that has been produced through pulverization by a pulverization apparatus in which an oxygen concentration in a pulverization atmosphere is controlled to be 5000 volume ppm or less from the pulverization apparatus into an airtight container without exposing the rare-earth alloy powder to an atmospheric air; supplying the rare-earth alloy powder from the container into an airtight feeder box without exposing the rare-earth alloy powder to the atmospheric air; filling the rare-earth alloy powder from the feeder box into a cavity formed in a die of a compacting apparatus; and making a compact of the rare-earth alloy powder through a pressing process.
In a preferred embodiment, the rare-earth alloy powder is supplied from the container into the feeder box through a hollow structure having an inert atmosphere therein.
In a preferred embodiment, the step of making a compact is performed in an air atmosphere.
An embodiment of the inventive powder-filling device includes: a feeder box having an enclosure forming an airtight space for containing a powder therein; a level sensor for measuring an upper surface level of the powder contained in the space; and powder supply means for supplying the powder into the space based on an output from the level sensor.
In a preferred embodiment, the powder-filling device further includes stirring means provided in the space.